That makes sense and should work for my case - thanks as always for the
quick and comprehensive answer, Kenton.
On Wed, Nov 22, 2017, at 02:53 PM, Kenton Varda wrote:
> Hi Liam,
>
> Currently there's no way to do this.
>
> But I think it would lead to a different kind of type unsafety anyway:
> If you accept a T::Builder, the implication is that it doesn't need to
> be the root builder of the message. If someone passed an inner object,
> or an orphan, what should happen?>
> What I'd suggest instead is that you encapsulate MessageBuilder into
> your own API, such that people can only construct a MessageBuilder of
> the right type. Two ways you might do that:>
> 1) You could define a type `FooMessage` which privately contains a
> MallocMessageBuilder and which has a getRoot() method that only
> returns the appropriate builder type. Then have your project
> require a `FooMessage` instead of a `MesasgeBuilder`. If you have a
> lot of root types, you could of course make this a template.>
> template <typename T>
> class TypedMessage {
> public:
> typename T::Builder getRoot() { return inner.getRoot<T>(); }
>
> private:
> capnp::MallocMessageBuilder inner;
> };
>
> 2) If your framework controls the scope in which the message is
> supposed to be constructed (like, your framework calls "up" to the
> application to request a message), then it could allocate the
> MessageBuilder internally and pass the T::Builder up to be filled
> in, so the app never sees the MessageBuilder. This is what Cap'n
> Proto RPC does, for example. This has the neat advantage that your
> framework can play with message allocation strategies. E.g. Cap'n
> Proto RPC aims to be able to allocate messages directly inside a
> shared memory segment, if using a shared memory transport, without
> the application knowing that this is the case.>
> -Kenton
>
> On Wed, Nov 22, 2017 at 1:10 PM, Liam Staskawicz
> <[email protected]> wrote:>> __
>> Hello,
>>
>> Part of my current project provides a mechanism to transmit capnp
>> messages and I'd like to accept a T::Builder as input, rather than a
>> MessageBuilder if possible, to help avoid messages of the incorrect
>> type being sent on a channel.>>
>> Is it possible to access the MessageBuilder that a T::Builder is
>> associated with through the T::Builder? Is there a better way to
>> approach this issue?>>
>> Thanks!
>>
>> Liam
>>
>>
>>
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